Treatment fluids required in treatment of a patient by continuous renal replacement therapy, hereinafter referred to as CRRT, must often be stored in a temperature which is relatively cold with respect to the patient's body temperature. Such fluids are typically stored at temperatures ranging from 2° to 20° Celsius in order to preserve the fluids in a state so that the function and integrity of the fluid is maintained. For this reason it is often desirable to heat the fluid to an appropriate temperature when introducing it into the patient's body to prevent any rapid decrease in the patient's body temperature. For the same reason it is desirable to heat fluids that are to be in contact with blood via a semi permeable membrane in a blood treatment unit or the blood as such before the blood is reintroduced to the patients body.
In dialysis treatment some heat is generally lost to the environment from the blood circulating in an extracorporeal circuit comprising a bloodline and a dialyzer in which the blood is treated. Heat loss from the blood in the extracorporeal circuit, in time, results in loss of heat from the patient's body.
The continuous nature of CRRT increases the potential of heat loss from the blood circulating in the extracorporeal circuit and the patient may, under certain circumstances, experience a depression of corporeal temperature. This is especially significant when the treatment fluid has a temperature lower than the extracorporeally circulated blood.
Loss of heat from the extracorporeally circulated blood is due to diffusion of heat either to the surrounding air or by diffusion or convection to the effluent fluid. Effluent fluid is constituted by the dialysis fluid used in the treatment in hemodialysis (HD) mode as well as the fluid extracted in hemodiafiltration (HDF) or hemofiltration (HF) mode. Effluent fluid is sent to a drain whereby the heat diffused from the blood to the effluent fluid is lost. Also the infusion of treatment fluid to the blood may result in decreased temperature of the blood.
Usually the main part of the heat in the blood is lost to the effluent fluid. A special challenge occurs during periods of low blood flow, about 50 ml/min, since the temperature decrease is larger compared with periods of medium blood flows, in the range of 100-200 ml/min, or high blood flows, in the range of 200-300 ml/min.
For this reason it is desirable in some CRRT treatments to compensate for, or to reduce, heat loss from the extracorporeally circulating blood.
In case the treatment fluids are stored sterile in flexible bags or rigid or semirigid containers it is a challenge to heat the treatment fluid by means of devices requiring direct contact with the fluid. To make sure that the extracorporeally circulated blood is not severely affected the temperature of any treatment fluid should not exceed 41° Celsius.
U.S. Pat. No. 6,349,170 discloses a renal replacement therapy system comprising a blood warmer capable of being attached to a renal replacement therapy monitor and a venous line adapted to be received by and cooperate with the blood warmer. The blood warmer comprises an external cylindrical surface. The venous line is engaged helically on the cylindrical surface serving as a heat exchanging section of the blood warmer. A clam-shell helical sleeve is installed over the heat exchange section to hold the venous line in place and to improve the heat transfer characteristics from the heat exchange section to the venous line.